The present invention generally relates to measuring the duration of an event and, more particularly, to providing a high resolution measurement of the duration of closely spaced events.
There are many instances when the duration of an event must be measured with a high precision where the duration is measured in nanoseconds. Event examples include high energy nuclear physics events and laser ranging. One type of straight-forward counter simply generates high frequency pulses and counts the pulses. The resolution is inversely proportional to the pulse frequency, e.g., .+-.10 ns for a 100 MHz clock and is too coarse for precision work.
An improved time interval resolution can be obtained by interpolating the time elapsed between clock pulses. In one interpolation technique, a vernier approach is used to generate interpolation intervals. For example, U.S. Pat. No. 4,433,919, issued Feb. 28, 1984, to Hoppe, teaches the use of tapped delay lines wherein the difference between a start/stop pulse and an internal clock pulse can be provided from the tap outputs and combined with a gross clock count to derive a high resolution measurement. The tapped delay lines are read after the event is completed so that event repetition is limited.
In another interpolation technique, time stretching is used, e.g., by capacitor charging and discharging. A capacitor may be charged through a low resistance circuit so that an appreciable charge can be stored during a pulse interval. The capacitor is thereafter discharged through a high resistance circuit to stretch the discharge time. The discharge time is a measure of the stored charge and, hence, the time during which the capacitor was charged. See, e.g., J. Kalisz et al., "Error Analysis and Design of the Nutt Time-Interval Digitiser with Picosecond Resolution," J. Phys. E:Sci. Instrum. 20:1330 (1987). The time for making a measurement must include the "stretched" time required to discharge the capacitors and obtain the desired delay information.
Yet another approach provides for charging/discharging capacitors during intervals corresponding to the time between start/stop pulses and system clock pulses. The voltage on the capacitor can then be converted to an interpolation interval. See, e.g., J. Kostamovaara et al., "Time-to-Digital Converter with an Analog Interpolation Circuit," Rev. Sci. Instrum. 57(11):2880 (1986).
The problem of obtaining repetitive high resolution time interval measurements is addressed by the present invention and it is an object of the present invention to provide a time interval measurement circuit for closely spaced events.
It is another object of the present invention to provide synchronous internal waveforms for both gross time interval determinations and start/stop interpolation time intervals.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.